It is known in the prior art that the pressure ratio (P.sub.IN /P.sub.OUT) between a foreline and a load port of a vacuum vapor pump having a source of vaporizable fluid, such as a diffusion pump, is a function of the power supplied by a heater to a bath of the vaporizable fluid. Such heaters are usually electrically powered from a unregulated, AC source. As the voltage of the unregulated source varies, the power supplied to the heater varies, with resulting changes in the ratio of P.sub.IN /P.sub.OUT. While it is possible to provide a regulated AC source for the power supplied to the heater, such sources are relatively expensive and frequently do not include provision for varying the power supplied to the heater. Thereby, accurate control of the pressure ratio P.sub.IN/P.sub.OUT is not generally attained with the prior art, and there are difficulties in changing a set point value for the pressure ratio.